The reactin of non-cognate complexes of aa-tRNA, EFTu, and GTP with U-programmed ribosomes is being investigated. TRNAs which read codons differing by only one base from UUU are the major subject of our investigations. We will investigate these reactions using ribosomes from wild-type and mutant E. coli. defective in protein biosynthesis. The reaction of poly U-programmed ribosomes with Leu-tRNA2 Leu ternary complex is being studied in the pre-steady state phase with a view to elucidating its mechanism.